Line biasing Flaws - need solutions

[Nigel Worsley]

A capacitor after the diode will sort out the impedance issue, it doesn't need to be particularly large as the impedance is only relevant for higher frequencies - 100nF would probably be enough. Add a zener across it to prevent it charging up too much and changing the bias voltage.

BTW, 3.3V tranzorbs ARE available, NXP, Vishay and ST all do them.

[berntd]

Originally Posted by Nigel Worsley

A capacitor after the diode will sort out the impedance issue, it doesn't need to be particularly large as the impedance is only relevant for higher frequencies - 100nF would probably be enough. Add a zener across it to prevent it charging up too much and changing the bias voltage.

BTW, 3.3V tranzorbs ARE available, NXP, Vishay and ST all do them.

Hello
I did look for tranzorbs but their breakdown voltage was not good enough at over 4V for most of them. The absolute maximum in my system is 4.0V for the processor.
I will check again.

The diode is the correct way to go here in my opinion, if we can make it work.

So we have 3V3, diode(a-k), pullup, dataline+.
Did you mean the capacitor needs to go from the 3V3 to gnd on the diode a side? How will this work if the diode is reverse biased?

Or do you mean the capacitor should go across the diode, providing an ac path to 3V3?

Kind regards
Bernt

[dangeross]

Another option might be to use a chip like the Intersil ISL3296E with series resistors to limit the drive current to provide the line bias. You could even only enable the bias while the Controller is expecting a response to save some power if you device is battery operated.